The present invention relates in general to a linear actuator. More specifically, the invention relates to a linear actuator that can be incorporated into a remote operation device that is mounted on a circuit breaker, such as a molded case circuit breaker or an earth leakage circuit breaker, wherein the linear actuator drives the handle of the circuit breaker to ON, OFF, and RESET positions by remote control operation.
Circuit breakers are used as components in low voltage distribution systems, are installed on switch boards or control boards, and constitute s of a centralized control system. When a circuit breaker is used in combination with a remote operation device, an external accessory device is provided that changes over the handle of the circuit breaker between ON, OFF, and RESET positions according to an external command (an electric signal). In addition to the traditional motor-driven system consisting of a drive motor, reduction gears, and a feed screw, a type of remote operation device is known and commercially available at present in which an electromagnetic linear actuator directly drives the operation handle. Examples of electromagnetic linear actuators are shown, for example, in Japanese Unexamined Patent Application Publication No. 2002-319504 (FIG. 1 and FIG. 6) and Japanese Unexamined Patent Application Publication No. 2006-40665 (FIG. 3).
The construction of a remote operation device for a circuit breaker and a circuit breaker used in combination with the remote operation device that are disclosed in the above-referenced patent applications will now be described with reference to FIGS. 6(a), 6(b), 6(c), 7, and 8.
In FIGS. 6(a), 6(b), and 6(c), the symbol 1 designates a circuit breaker (a molded case circuit breaker), the symbol 1a designates a rocker type open and close operation handle projecting outward and upwards from a cover of the case 1b of the breaker, and the symbol 2 designates a remote operation device mounted on the top of the circuit breaker 1 and linked to the handle 1a. The remote operation device 2 is equipped with an electromagnetic linear actuator in a case 3. The linear actuator is composed of a guide rail 4, a moving part 5 guided and supported by the guide rail 4 (details of the moving part will be described later), and a pair of stationary parts disposed at both sides of the moving path of the moving part, each stationary part consisting of an E-shaped magnetic yoke 6a and an actuation coil 6b wound around the yoke 6a. In the arrangement where the remote operation device 2 is mounted on the circuit breaker 1, the operation handle 1a of the circuit breaker is engaged and linked with a recess structure formed on the bottom surface of the moving part 5. The reference numeral 7 designates a handle for manual operation provided on the moving part 5, and the reference numeral 8 designates an auxiliary cover provided on the top surface of the case 3.
As shown in FIG. 7, the handle 1a of the circuit breaker 1 links to a movable contact of a main circuit contact structure 1d through a switching mechanism 1c assembling a toggle linkage and a toggle spring. The switching mechanism 1c links to an overcurrent trip device 1e through a latch mechanism. When the circuit breaker is used alone and the handle 1a is moved from OFF position to ON position or ON position to OFF position indicated in FIG. 7, the switching mechanism 1c makes a reversing motion according to the movement of the handle 1a (the toggle linkage is urged by the toggle spring), to close or open the main circuit contact. When the overcurrent trip device 1e detects overcurrent in the main circuit and releases the latch mechanism, the circuit breaker trips to open the main circuit contact and the handle 1a stops at a TRIP position in the middle between ON and OFF positions on account of a condition of balance between the toggle linkage and the toggle spring, indicating trip action. To turn on the main circuit contact again after the trip action, the handle 1a is once again moved from TRIP position to RESET position to reset the latch mechanism, and then moved to ON position to close the main circuit contact.
FIG. 8 shows a structure of the electromagnetic linear actuator of a prior art. The moving part 5, which is guided and supported movably in the direction of the arrow, has permanent magnets 9 for field system on the left and right side surfaces opposing stationary parts 6. The stationary part 6 composes a coil section consisting of an E-shaped magnetic yoke 6a and an actuation coil 6b wound around the center leg of the yoke. The length A of the permanent magnet 9 (a distance between N and S poles) and the distance B between the center leg and an outer leg of the magnetic yoke 6a are set to satisfy the relation B<A<2B.
In operation of the linear actuator of the above-described structure, when an operation command for ON or OFF is given from outside to the remote operation device 2 shown in FIG. 6, and an excitation current is fed to the actuation coil 6b of the linear actuator according to the command, a magnetic thrust is generated between the magnetic yoke 6a of the stationary part 6 and the permanent magnet 9 of the moving part 5 corresponding to the direction of the excitation current. The magnetic thrust moves the moving part 5 from one stroke end to the other stroke end along the guide rail 4, thereby changing over the rocker handle 1a of the circuit breaker 1 to ON or OFF position. A switch for detecting ON or OFF position is provided at the end position of moving path of the moving part 5. When the moving part 5 reaches the end position of ON or OFF in ON or OFF operation of the circuit breaker 1, the excitation of the actuation coil 6b is stopped according to the output signal from the switch for position detection. In the condition without excitation of the actuation coil 6b, the moving part 5 receives magnetic force from the permanent magnets 9 and attracted, and held at the end position corresponding to ON or OFF position.
FIG. 9 shows an example of characteristics of thrust and load when a remote operation device 2 having the above-described construction is mounted on a circuit breaker 1 (a low rating molded case circuit breaker) and the handle 1a is driven to ON, OFF, or RESET position by the linear actuator. In FIG. 9, the abscissa represents a stroke (mm) to ON and OFF directions relative to the center of the handle 1a and the ordinate represents a thrust (N) (in which + means a thrust in the ON direction and − means a thrust in the OFF direction). The characteristic curves A, B, and C represent the loads (primarily, a reaction force from the toggle spring provided in the switching mechanism) exerted on the linear actuator of the remote operation device 2 from the handle 1a of the circuit breaker 1 in the process of ON operation, OFF operation, and RESET operation, respectively, of the circuit breaker. The characteristic curves D and E represent electromagnetic thrust forces on the moving part 5 when excitation current (direct current) for ON and OFF directions, respectively, is fed to the actuation coil 6b (FIG. 8) of the linear actuator.
In these characteristic curves, the area enclosed by the abscissa of thrust=0 and each of the characteristic curves A through E represents a work done in each operation process. To change-over the operation handle of the circuit breaker between ON, OFF, and RESET positions, the electromagnetic thrust (characteristic curves D and E) must overcome the load of the operation handle (characteristic curves A, B, and C) during the change-over process. With regard to this point, in the process of changing-over the handle of the circuit breaker from ON to OFF, the load increases accompanying the handle motion (characteristic curve B), reaches a peak just after passing the point of stroke=0, and then abruptly drops because of reversal action of the switching mechanism. On the other hand, the electromagnetic thrust of the linear actuator (characteristic curve E), starting excitation of the actuation coil at ON position of the handle, increases gradually in the first half of the change-over process, accelerating the moving part 5 and putting an inertial force on the moving part. The moving part 5 goes over the peak point of the load (characteristic curve B) in the latter half of the charge-over process, and after that, rushes to the OFF end position to finish the OFF operation of the handle.
The process to change-over the handle of the circuit breaker from OFF to ON is approximately the same as the process from ON to OFF described above. In the latter half of the operation process, the load (characteristic curve A) abruptly drops on reversal action of the switching mechanism. The moving part of the linear actuator accelerates from the start of the process receiving the electromagnetic thrust (characteristic curve D), being added by an inertial force, rush to the ON end position to finish ON operation of the handle of the circuit breaker.
In a trip action of the circuit breaker (in which the actuation coils of the linear actuator are in a condition without excitation), the moving part 5 of the linear actuator coupled to the operation handle are attracted by the magnetic force of the permanent magnets 9 and held at the ON side. So, the handle 1a of the circuit breaker does not move to TRIP position like the case of single use of the breaker, but stays still at around the ON position. To reset the switching mechanism using the remote operation device after trip action of the circuit breaker, the handle is once again moved back to the ON end position by operating the linear actuator, and then moved anew from this ON position towards OFF position to make reset operation of the switching mechanism. In the process of this reset operation, the load (characteristic curve C) increases near the stroke end of OFF side to anchor the latch of the switching mechanism at a lock position. In this respect, the moving part, started at the ON position and traveled to the OFF end position, can overcome the load (characteristic curve C) with the aid of accompanied abundant inertial force and arrives at the RESET position.
A remote operation device that changes-over a handle of a circuit breaker using the linear actuator described above has the following problems in operation and functional performances. When an excitation current is fed to the actuation coils 6b, according to an operation command in a linear actuator having the conventional structure shown in FIG. 8, the moving part 5 moves at once from one end position to the other end position receiving the electromagnetic thrust. The moving part 5 is accelerated with the traveled distance, and with the aid of the inertial force, the moving velocity of the moving part 5 increases. On the other hand, the load of the handle (FIG. 9) abruptly falls down just before the end position as described previously. Consequently, the handle that reached the ON or OFF end position violently collides against the window frame formed on the cover 1b (FIG. 7) of the breaker case. The impulsive force might break the handle, which is made of a resin. To cope with this problem, a remote operation device has been provided with a dumping structure at the stroke end of the moving part, thereby absorbing the impulsive load of the handle and avoiding breakdown of the handle. This structure, however, results in increased complexity and higher cost of the device.